An ITO film is widely used as a transparent electrode (film) of a display device; mainly a liquid crystal display. As a method of forming this ITO film, ordinarily employed is a method generally referred to as a physical vapor deposition method such as the vacuum deposition method or sputtering method. Particularly, the magnetron sputtering method is often employed for forming such ITO film from the perspective of operability and stability of the film.
The formation of film with the sputtering method is performed by physically colliding a positive ion such as Ar ion to a target established in the negative electrode, discharging materials composing the target with such collision energy, and laminating films of approximately the same composition as the target material on the substrate on the opposing positive electrode side.
The coating method employing the sputtering method is characterized in that it is capable of forming a thin film of angstrom units to a thick film of several ten μm at a stable deposition speed by adjusting the processing time, power supply, and so on.
A particular problem in forming an ITO film is the sputtering target density and generation of nodules during the sputtering operation. The sputtering target for forming the ITO film is prepared by sintering the powder in which tin oxide powder and indium oxide powder are mixed at a prescribed ratio. Nevertheless, since this powder is of originally differing components and composition, the grain diameter of such powder will vary, and it is inevitable that this powder will tend to be inferior at the stage of mixing in comparison to powders having the same component.
Nonetheless, since recent electronic devices are being further downsized and miniaturized, the transparent conductive film itself is being thinly miniaturized, and, if such film is not formed uniformly, there is a trend of the influence on the deterioration of quality becoming great.
In consideration of the above, as a result of examining the pores inside the sintered body, it has become evident that tin oxide is segregated on the inner walls of the pores and the periphery thereof. And, one factor of such segregation being generated is that tin oxide, which is the ITO material, was aggregated.
Therefore, the improvement in the dispersibility of tin will lead to the reduction of pores, and it is anticipated that the effective manufacture of ITO sintered body targets with few generation of nodules will become possible.
From this perspective, for the purpose of improving the dispersion status of tin, considered may be a method of premixing indium salt and tin salt, adding a precipitant to this mixed solution, simultaneously obtaining mixed sediments (coprecipitation method) via the neutralization process of indium hydroxide and metastannate, and drying, washing, sintering (oxidizing) this to obtain a mixed powder of indium oxide and tin oxide.
Although this coprecipitation method, which is considered to improve the dispersibility most efficiently, certainly makes the dispersibility of tin favorable, there is a problem in that the production cost will increase since there are numerous processes in comparison to ordinary manufacturing methods.
When the number of nodules on the erosion face of the ITO target becomes large, this induces irregular sputtering, and, in some cases, there is a problem in that an abnormal discharge or clustered (solidified) film will be formed, which may become a factor of a short circuit.
Simultaneously, oversized grains (particles) may float inside the sputtering chamber, and there is a problem in that this may similarly cause a short circuit in the thin film circuit by re-attaching on the substrate and cause protrusions on the thin film.
Accordingly, although it was necessary to obtain a high-density sintered body target having uniform components, there was a problem in that a manufacturing method of ITO powder and a manufacturing method of an ITO target capable of satisfying these requirements and enabling low manufacturing costs could not be obtained.